In drilling, a lubricating oil is generally applied over that portion of a workpiece to be drilled in order to reduce friction between the workpiece and the drilling tool or bit and improve workability. For accurate drilling, the center of the cutter is generally aligned with a punch mark previously set on the workpiece to mark the exact location desired to be drilled.
Prior art drilling tools, as generally depicted in FIG. 5, utilize a hollow shaft 4 for supporting an annular cutter 3 over a workpiece 1. Engaging member 6 abuts the upper end of a pilot pin 5 which is initially mounted within cutter 3 in a vertically movable manner. When the cutter 3 is lowered toward the workpiece 1, there is a change in the distance L between an engaging member 6 and pin 9 horizontally fixed to an outer cylinder 8 passing through hollow shaft 4 and supported by bearing member 7. Such a change does not permit the pin 9 to act on a spring 10, so that the spring 10 temporarily loses its resilient force. Without the force of the spring 10 to bias it against pin 9, it is difficult to maintain engagement of the lower end of the pilot pin 5 within the punch mark 2 provided on the workpiece. Consequently, the pilot pin 5 may readily disengage the mark 2. Unsatisfactory engagement or entire disengagement of the pilot pin 5 causes cutting oil (not shown) to flow down onto the workpiece through groove 11 of the pilot pin in the form of droplets. Such oil droplets have to be wiped away, leading to troublesome alignment of the pilot pin 5 with the punch mark and wasteful use of the cutting oil.
In view of the above-mentioned disadvantages of conventional drilling machines, the present invention has for its object, the prevention of disengagement of the pilot pin for more accurate and facile drilling. A further object of the subject invention is the prevention of oil leakage prior to drilling while facilitating the alignment of the pilot pin within the punch mark.